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Bike backfiring after rebuild

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So I just got done rebuilding my 05 crf250r and couldn't get it start by kicking or even push starting. I had to pull start it with my truck which still took a few tries but when it finally started, it was backfiring like crazy. I figured timing might be off so I took it apart again and made sure it was right. Then it still wouldn't start and I had to pull it again. This time, it didn't backfire when it was first started but it did after idling and revving for a bit. I don't think there's anyway the timing is causing the problem as I made sure to get it dead on. What else could it be? Here's what I replaced:

-Valves and guides (kibblewhite & were shimmed by my local shop while the head was still off the bike)

-piston and rings (pro x)

-camshaft (wiseco)

-also, new spark plug, new air and oil filters, cleane carb, fresh gas and oil

Any ideas as to what could be causing this?

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How did they shim it with the head off? Did you re-check you valves?

I did not, I assumed they got it right. They had the head doing machine work on it and went ahead and shimmed them. Does it have to be on the bike with the cam chain attached to shim them? This is my first four stroke so I wasn't positive, but I assumed the shop would know. Edited by onealmxwilson

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Any time you re-shim a valve you need to turn the motor over at least a few times to get everything seated correctly, especially if you removed/replaced the valves and springs.

 

For future reference, if your bike won't start after a rebuild never try to pull start it with another vehicle.   If it’s not starting, there’s usually a good reason why.  Also, if your timing is off you can destroy your valves very quickly by them hitting the piston.

 

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